The present invention relates to a method of controlling the extrusion of a multilayer parison.
A method of blow molding is used to manufacture a hollow container made of a resin. In manufacturing such a container, a parison is injected in a die, and a pressurized gas is then blown into the parison to thereby form the container having a shape corresponding to that of the cavity of the die.
Japan Patent Application (OPI) No. 99115/87 (the term "OPI" as used herein means "unexamined published application") discloses a multilayer parison extruder in which an annular member is concentrically provided in an annular passage through which a main resin flows. An auxiliary resin is injected from one or more annular nozzles of the annular member. Each of the nozzles is provided with a material inlet port. Different materials including the main and the auxiliary resins are injected from the injectors of the extruder so that the materials are stored in the accumulators of the extruder, respectively. The material inlet ports for the nozzles are connected to the accumulators. The accumulators are activated to force the materials through the annular passage and the annular nozzles so that the materials constitute a multilayer parison as they are extruded by the extruder. For example, the multilayer parison consists of innermost and outermost layers of the main resin which is a high-density polyethylene having a good molding property but which is permeable to gasoline, a central layer of the auxiliary resin which is a nylon resin and which is impermeable to gasoline, and inner and outer intermediate layers consisting of an adhesive for conjoining the central layer to the innermost and the outermost layers. Thus, a gasoline tank is made from the parison in the conventional methods of blow molding.
However, there is a problem in that the central layer of the auxiliary resin is likely to be cut off halfway or to expand forming an expansion 20c.sub.1, as shown in FIG. 5. More specifically, a pressure difference exists between the inner and the outer intermediate layers of the adhesive while the accumulators are activated to force the materials therefrom. Thus, the central layer of the auxiliary resin is pushed from the higher-pressure layer of the adhesive toward the lower-pressure layer of the adhesive so that the central layer is deformed so as to decrease the thickness to an unacceptable low value, or in an extreme case, to zero. Such a phenomenon can also happen if a pressure difference exists between the innermost and the outermost layers of the main resin. If the pressure difference is 20 kg/cm.sup.2 or more and the parison is for a motor vehicle's gasoline tank, the central layer of the auxiliary resin is cut off or ruptured.
Further, since the nylon resin for the central layer is higher in viscosity than the other materials, the pressure drop of the nylon resin lags behind that of the other materials at the end of the injection of the resin and the other materials from the accumulators into the annular passage and the annular nozzles even if the injection is stopped simultaneously for all of the resin and the other materials. Therefore, it is likely that only the nylon resin will continue to be injected from an auxiliary resin passage after the injection of all the materials has been terminated so that the central layer of the nylon resin expands to form the expansion 20c.sub.1. If the parison has such an expansion, it is defective because it does not have a prescribed thickness ratio at the area of expansion. Injection pressure much lower than the injection pressure which acts on each material when the accumulator is put in action to push out the material acts from the injector to the material at an annular outlet port while the material pushed out from the injector is stored in the accumulator after the previous pushing-out of the material from the accumulator. Besides, the injection pressure for the nylon resin, which is higher in viscosity than the other materials, needs to be higher than the injection pressure for the other materials. Therefore, only the nylon resin is likely to be forced out in a small quantity from the annular outlet port during the storage of the resin in the accumulator so as to create a similar expansion near the port. A multilayer parison having such an expansion, is defective since it does not have the prescribed thickness ratio at the expansion. This is also a problem.